and play a major role in the work of breathing (see ). All lung volumes are subdivisions of total lung capacity (TLC), the total volume of air that is contained in the lung at the point of maximal inspiration. Lung volumes are reported in liters either as volumes or as capacities. A capacity is composed of two or more volumes. Many lung volumes are measured with a spirometer. The patient is asked to first breathe normally into the spirometer, and the volume of air (the tidal volume [VT]) that is moved into and out of the lungs with each quiet breath is measured. The patient then inhales maximally and exhales forcefully and completely, and the volume of exhaled air is measured. The total volume of exhaled air, from a maximal inspiration to a maximal exhalation, is the vital capacity (VC). Residual volume (RV) is the air remaining in the lungs after a complete exhalation. TLC is the sum of VC and RV; it is the total volume of air contained in the lungs at the end of maximal inspiration, and it includes the volume of air that can be moved (VC) and the volume of air that is always present (trapped) in the lungs (RV). Functional residual capacity (FRC) is the volume of air in the lungs at the end of exhalation during quiet breathing and is also called the resting volume of the lungs. FRC is composed of RV and the expiratory reserve volume (the volume of air that can be exhaled from FRC to RV).
Measurement of Lung Volumes
RV and TLC can be measured in two ways: by helium dilution and by body plethysmography. Both methods are used clinically and provide valuable information about lung function and lung disease. The helium dilution technique is the older and simpler method, but it is often less accurate than body plethysmography, which requires sophisticated and expensive equipment.
21.1
Pulmonaryfunctiontestsareoftenusedtodiagnoseabnormalitiesinlungfunctionandtoassesstheprogressionoflungdisease.Theycandistinguishthetwomajortypesofpulmonarypathophysiologicprocesses:obstructivelungdiseasesandrestrictivelungdiseases.Forexample,innormalindividuals,theratioofRVtoTLCislessthan 0.25.Thusinahealthyindividual,approximately25%ofthetotalvolumeofairinthelungistrapped.Inobstructive pulmonary diseases, anelevationinRV/TLCratioissecondarytoanincreaseinRVoutofproportiontoanyincreaseinTLC.Incontrast,inrestrictive lung diseases, theelevationintheRV/TLCratioiscausedbyadecreaseinTLC.
In normal individuals, the FRC measured by helium dilution and the FRC measured by plethysmography are the same (
Table 21.1 ). This is not true in individuals with lung disease. The FRC measured by helium dilution is the volume of gas in the lung that communicates with the •Fig. 21.4 Measurement of Lung Volume by Helium Dilution. C1,knownconcentrationofaninertgas;C2,new(previouslyunknown)concentrationofthegas;V1,knownvolumeofabox;V2,lungvolume(initiallyunknown).
airways, whereas the FRC measured by plethysmography is the total volume of gas in the lungs at the end of a normal exhalation. If a significant amount of gas is trapped in the lungs (because of premature airway closure; see ), the FRC determined by plethysmography is considerably higher than that determined by helium dilution.